Low voltage liquid crystal display

ABSTRACT

An improved orientation type liquid crystal display having a layer of a mixture of liquid crystal materials confined between a pair of substrates and electrodes for applying an electric field across the substrate. The mixture of liquid crystal materials includes between 5 and 60 percent by weight of an orientationtype liquid crystal material and between 95 and 45 percent by weight of a dynamic scattering type liquid crystal material.

United States Patent [191 Hsieh June 11, 1974 LOW VOLTAGE LIQUID CRYSTAL DISPLAY [75] Inventor: Paul Y. Hsieh, Ridgewood, NJ.

[73] Assignee: Ing. C. Olivetti & C., S.p.A., lvrea,

Italy [22] Filed: Aug. 9, 1972 [21] Appl. No.: 279,130

[52] U.S. Cl. 350/160 LC, 23/230 LC, 252/408, 350/150 [51] Int. Cl G02f l/28, G02n 31/00 [58] Field of Search 252/408; 350/160 LC, 150; 23/230 LC [56] References Cited UNITED STATES PATENTS 3,689,525 9/1972 Scheurle et a1. 260/473 R 3.690.745 9/1972 Jones 350/160 R FOREIGN PATENTS OR APPLICATIONS 1,170,486 1 H1969 Great Britain 252/408 OTHER PUBLICATIONS UsolTseva et al., Chemical Characteristics, Structure AND and of Liquid Crystal, Russian Chem., Rev., Vol. 32, No. 9, pp. 495-507, (September 1963).

Primary Examiner-William J. Van Balen [57] ABSTRACT An improved orientation type liquid crystal display having a layer of a mixture of liquid crystal materials 1 Claim, No Drawings 1 LOW VOLTAGE LIQUID CRYSTAL DISPLAY BACKGROUND OF THE INVENTION This invention relates generally to improved low volt- 5 age liquid crystal materials and more particularly to improved low voltage liquid crystal materials which are usable at room temperature and below.

Because of their uniqueproperties, liquid crystal displays are becoming widely used in many applications such as calculators and. watches. Most'liquid crystal materials used in'displays to date operate in the dynamic scatteringv mode. These. materials are normally transparent to incident light but become turbulent under the: influence: of an applied'electricfield and scatter incident'light.

Normally, dynamic scattering type displays operate with AC voltages on theorder'of to 30 volts. DC Eiritation voltages of thesameworder of the magnitude may also be used but greatly reduce-:the-lifeofthe. liquidv crystal material.

Recently nematicliquidJcrystal materials which 'oper-'- ate on much'lower voltage'levels, for instance'on'the:

order of 6 to 8volts for Ato 1 mil cells, have'been introduced, suchas those described.in'thearticleentitled -Voltage Dependent Optical Activity of a Twisted'Nematic Liquid Crystal" which appearsin' the. Feb:

1971 issue of Applied Physics-Letters; Indisplays using these low voltage materials .theinteriorfaces' of the sub strates between which'the liquidzcrystal. material confined are treated by rubbingorin'some'other manner to cause themolecules of theliquidcrystalmaterial in the vicinity of the substrate:to alignithemselves parallel to each other and to the plane:ofthe:substrate. The

substrates are arranged so that the molecules ofa liquid crystal material adjacent to one. of the-substrates are aligned peripendicular to the molecules ofthematerial adjacent to the other substrate. The intermediate -molecules of the layer of liquid crystal material then tend to align themselves in acorkscrew fashion between the substrates so that they twist the axis ofpolarization 'of light passing through the layer of material by 90.

Under the influence of anelectricfield perpendicular to thelayer, however, the-:moleculesof theliquid crys tal material tend to align themselves parallel tothe-field so that light passingthrough portions of thedisplay ex-- cited by an electric field is-not'twisted but passes through unaltered. Thusitis possible-withthe use of polarized. layers to produce. either whiteon-blackor' black-on-white displays.

These orientation-type liquid crystal displays have significant advantages overthe'dynamicscattering type:

SUMMARY OF TI-IEEINVENTION Inorder to overcome these problems which make the previously known orientation-type liquidcrystal materials impracticalfor most uses, applicant provides an orientation-typeliquid. crystalmaterial which is comprised of a mixtureof an orientation=typeliquid crystal material with" a. dynamicscattering type liquid crystal material. Theresulting.liquid crystal material is of the orientation-type: but has a. crystal-nematic transition temperature-on theorder'of or below that of the component dynamic scatteringtype liquid crystal material.

DETAILED DESCRIPTION Most nematic liquid crystal materials have a general structure WherewX and Y are a groupselected from CN, R, R0, ROCOO and 'ROCO (R'is an alkyl group of one to eight carbon atoms), andZQmay beselected from CH ="N, N'="N,-

and CH= CH: The mbifies of liquidcrystal materials which operatein' the dynamic scatteringmode displays. As stated. above, the: voltage levels required for exitation of thedisplay, onthe order of 6 to 8volts, are much lower than those-required for'the'displays operating in the dynamic-scattering mode, thus making;

the displays directly compatible. with MOS-circuitry. Furthermore, the: contrast of these. displays. is signifi cantly improved'over that achievable withthe:dynamic scattering type displaysand their'powerconsumption is lower.

Known orientation-type liquid crystal materials have:

only very limited. usefulness, however; sincethey have relatively high mesophasetemperature and'donot operate at room temperature. BritishPat; No. 1,228,606 describes a number of orientation-type crystal materials and states that the-"crystalnematic-transition tem- (e.g., whereX RO and Y =R) have a negative dielectric anisotropy, i.e., thedielectric constant (e L perpendicular to the :axis of the-moleculeis larger than that parallel to thev molecular'axise Because'themolecules are-long and thin; themagnitude of the dielectric anisotropy of dynamic scatteringtype materials is quite small, generally onf'the order' of '(e =0.l to 0,2 and almost neverlarger than 3 in" magnitude.

Molecules ofliquidcrystals which operate .in the orientation mode (i.e., X RO'or'R, and Y ''-'C N) have a positive dielectric anisotropy i.e., the dielectric constant'parallel to axisofthe molicules (e is larger thanthat perpendicular to the molecular axis (a i Themagnitude'zof the dielectric anisotropy for orientation typeliquidicrystals is relatively larg. Onthe order ofe -"e '10 30.

Materials withhigher molecular dielectric anisotropy generally havehigher thermal stability and therefore higher crystal-nematic transition temperatures. This is apparent by comparing; 4-ethoxy benzylidene 4'-nbutylaniline (EBBA), adynamicscattering type liquid crystal materialhaving negative dielectric anisotropy of 6 -6 ="-0.l and alnematic range of 35 to C, with 4-ethoxybenzylidene 4'-amino-benzonitrile (PEBAB), an orientation-type liquid crystal material having a positive molecular dielectric anisotropy of e e =l7-2l depending upon frequency and temperature used in the measurements and a nematic range of 105 to 130C.

Applicant has found according to his invention that the mixture according to the invention may be either a single compound or a mixture of two or more of these compounds.

Examples of dynamic scattering type liquid crystal materials which may be used as a component in the mixture according to the invention are the following:

4-butyl, 4-ethoxy azoxybenzene by mixing an orientation-type liquid crystal material having a high positive molecular dielectric anisotropy with a dynamic scattering type liquid crystal material having a relatively low negative dielectric anisotropy,

a liquid crystal material results having a crystal-nematic transition temperature which is generally lower than that of the dynamic scattering type material for the preferred composition ratio but which operates as an orientation-type material.

Since the magnitude of the net positive molecular dielectric anisotropy, e c of the orientation-type material is much larger than the net negative one of the dynamic scattering type material, the mixture can consist of as much as 90 to 95 percent by weight of the lower temperature dynamic scattering type liquid crys- 5 mixture of orientation and dynamic scattering type liquid crystal materials operates in the orientation mode varies somewhat with the materials used, but is generally on the order of between 5 and 60 percent by weight of orientation-type material. The preferred range, however, is between about 10 to 30 percent. With high concentrations of orientation-type material there is a high nematic to liquid transition temperature and a higher viscosity. The nematic range and preferred concentration for a mixture of particular orientation and dynamic scattering type liquid crystal materials may be easily determined by a simple experimentation. Typical orientation-type liquid crystal materials are the following:

The above named liquid, crystal materials are known in the literature and may be easily obtained or produced. The method of preparation of liquid crystal display devices is also well known in the art. Display devices which use the compositions prepared in accordance with the present invention in the orientation mode with low driving voltages, for example, an order of 6-8 volts for A to 1 mil cells. These displays are operable at room temperatures and even below.

The following examples, all of which respond to electric fields in the orientation mode in their nematic range, are given to illustrate specific embodiments of the present invention.

EXAMPLE 1 A low voltage liquid crystal display was fabricated using to 1 mil thick layer of an orientatiomtype liquid crystal material mixture having the following composition:

Parts Components by Weight p-(p-ethoxy benzylidene) aminobenzonitrile 1 p-(p-n-but0xy benzylidene aminobenzonitrile 1 4-methoxy, 4'nbutyl benzylidene aniline 3.5 4-ethoxy. 4n-butyl benzylidene aniline 3.5

Other examples of orientation-type liquid crystal materials are given in the above mentioned British Patent and article appearing in the Applied Physics Letters. As is apparent from the article and the British patent, the orientation-type liquid crystal material component of manner described above and requires switching voltage 5 of between 6 and 8 volts,

EXAMPLE 2 A low voltage liquid crystal display similar to that described in Example I was prepared using a mixture having the following composition:

Parts EXAMPLE 6 A low voltage liquid crystal display similar to that described in Example 1 was prepared using a mixture having the following composition:

. 5 Parts Components by Weight Components by weight p-(p'n-butoxy benzylidene) aminobenzonitrile l p-(p-n-hexyl benzylidene) aminobenzonitrile l f benzyl'denle) ammobenzommle I p-(p-octanoyloxy benzylidene) aminobenzonitrile l A mmure of 4 'methoxy mi/benzene 4-methoxy, 4n-butyl benzylidene aniline L5 and 'y 4 y azflxybenlene l 4-ethoxy, fi'n-butyl benzylidene aniline 1.5 The mixture is nematic between 0C and 90C.

The mixture has a nematic range of between 16 and 83C andresponds well to electric fields in the orienta- EXAMPLE 7 tron mode In thls temperature ang 1 5 A low voltage liquid crystal display similar to that described in Example 1 was prepared using a mixture hav- EXAMPLE 3 ing the following composition:

A low voltage liquid crystal display similar to that described in Example 1 was prepared using a mixture hav- Pans ing the following composition: 20 Components by Weight p-(p-n-butoxy benzylidene) aminobenzonitrile l pans p-(p-n-hexyl benzylidene) aminobenzonitrile 1 Components by weigh A mixture of 4-butyl, 4'-methoxy azoxybenzene and 4-butyl, 4'-ethoxy azoxybenzene l p (p ethoxy benzylidene) aminobenzonitrile 1 The mixture IS nematic between 23C and 96C. p-(p-n-butoxy benzylidene) aminobenzonitrile l n-(O-hydroxy. p-methoxy henzylidene)-p-butyl 4. l 6

aniline n-(O-hydroxy, p-n-butyloxy benzylidene)-p- 3.84

butyl aniline EXAMPLE 8 0 A low voltage liquid crystal display similar to that de- The mixture is a free flowing fluid nematic at room scnbedm Example 1 was Pf usmgamlxmre temperature. It has a nematic liquid transition temperathe following. composltloni ture of 72C and gelled instead of crystallized upon cooling to 0C. it responds to electric fields even at a par gelled stage although at a slower speed due to the m- Cowman", by wcigm crease in viscosity.

I p-(p-n-butoxy benzylidcne) aminobenzonilrile l p-(p-n-hexyl benzylidene) aminobenzonitrile 1 EXAMPLE 4 A mixture of 4-butyl, 4-methoxy azoxybenzene v 4 and 4-butyl, 4'-ethoxy azoxybenzene 2 A low voltage liquid crystal display similar to that de- 0 scribed in Example I was prepared using a mixture havg the followmg composltloni The mixture has a nematic to liquid transition temperature of 94C and gells rather than crystallizing upon O Pans cooling to 0 C. t b Wei ht y 2 EXAMPLE 9 4 1 b I l 23.15;.fi Y.Lf,{ZZ'1Z2i;$;1;5 n e A low voltage liquid crystal display similar to that de- The mixture has a nematic range of between 5C and 85C. ib d i E l 1 w par d si g a ixt r haying the following composition:

EXAMPLE 5 Parts A low voltage liquid crystal display similar to that de- Components by Weight scribed in Example 1 was prepared using a mixture havp-(p-n-hutoxy benzylidene) aminobenzonitrile 3 mg the following composltlon' p-(p-n-hexyl benzylidene) aminobenzonitrile 3 A mixture of 4-butyl, 4'-methoxy azoxybenzene and 4-butyl, 4'-ethoxy azoxybenzene 14 Parts Components by Weight p-(p-nxy en ylidene) amin henzv r ii ril l The mixture has a nematic to liquid transition tempera- A mixture of 4-butyl, 4'-methoxy azoxy enzene O and 44mm azoxyhenzene l ture of 86(3 and gells rather than crystallizing upon The mixture is nematic between l4C and 94C. cooling [0 0 C.

EXAMPLE 10 A low voltage liquid crystal display similar to that described in Example 1 was prepared using a mixture having the following composition:

Parts Parts Components by Weight Components by Weight p-(p-n-butoxy benzylidene) aminobenzonitrile l p-(p-ethoxy benzylidene) aminobenzonitrile 3 p-(p-n-hexyl benzylidene) aminobenzonitrile l p-(p-n-butoxy benzylidene) aminobenzonitrile 3 p-(p-octanoyloxy benzylidene) aminobenzonitrile l A mixture of 4-butyl. 4-methoxy azoxybenzene A mixture of 4-butyl. 4'-methoxy azoxybenzene and 4-butyl, 4'-ethoxy azoxybenzene 20 and 4-butyl, 4'-ethoxy azoxybenzene 30 The g has a nemanc to liquid tempera 10 The mixture has a nematic to liquid transition temperature of 97 C gens upon cooling to 60 It does ture of 90C and upon cooling to 0C, gells instead of not have nemauc to crystal temperature crystallizing. The mixture continues to respond to elec- EXAMPLE 11 tric fields in the gelled stay even at -2()C, although at a slower speed due to the increase in viscosity.

A low voltage liquid crystal disp y F that 15 The dynamic scattering type liquid crystal material scribed in Example 1 was prepared usmgamrxture havgiven as a mixture of 4 butyl 4, meth0Xy azoxybem mg followmg composltlon: zene and 4-butyl, 4'-ethoxy azoxybenzene is comercially available from E. M. Laboratories, Elms Ford,

Pans N.Y. under the trade name MERCK PHASE V. Components by Weight The above listed examples on liquid crystal materials are not meant to be exclusive. They are merely the ones p-(p-n-butoxy benzylidene) ammobenzommle l benzylidene) aminObenmnm-ne 1 which the applicant has actually tried. More slgnifip-(p octanoyloxy benzylidene) aminohenzonitrile l tl however, li t h f d th t every i A mlxture ol' 4-butyl, 4 -methoxy azoxybenzene and 44mm tummy mxybenzene 3 ture of llqUld crystal material made in accordance with the principles of the invention has operated in the orientation mode and he has not found any nematic liquid The mixture has a neurnatic range from lfif to 93 C. crystal materials which do not work when mixed with other materials in accordance with the invention. EXAMPLE 12 What is claimed is:

A low voltage liquid crystal display similar to that de- 1, An orientation-type crystal display having a layer scribed in Example 1 was prepared using amixture havof a mixture of liquid crystal materials confined being the following composition: tween a pair of substrates and means for applying an electric field across said layer, said mixture of liquid crystal materials comprising:

componems fit' f between 5 and 60 percent by weight of orientationtype liquid crystal material consisting essentially of amin9bemmmlle. l a mixture of substantially equal parts of at least p-(p-n-butoxy benzylidene) aminobenzonltnle l p-(p-n-hexyl benzylidene) aminobenzonitrile l three compounds chosen from the group conslstmg p-(p octanoyloxy benzylidene) aminobenzonitrile l of;

3: 3 ffg g f jliggbi 'g'gtxg 1233 40 40 p-(p-n-butoxy benzylidene) aminobenzonitrile,

p-(p-n-hexyl benzylidene) aminobenzonitrile, p-(p-n-ethoxy benzylidene) aminobenzonitrile, and

The mixture has a nematic to liquid transition temperature of 72C and gells upon cooling to 60C but app-(p-octanoyloxy benzylidene) aminobenzonitrile,

parently has no nematic to crystal transition temperaand ture. between 95 and 40 percent by weight of a dynamic scattering type liquid crystal material consisting es- EXAMPLE sentially of a mixture of substantially equal parts of A low voltage liquid crystal display similar to that de- 4-butyl, 4-methoxy azoxybenzene and 4-butyl, 4-

scribed in Example 1 was prepared using a mixture havethoxy azoxybenzene.

ing the following composition: 

